Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing AlO2− and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions
Ideally, zeolites can be represented on an anhydrous basis, by the empirical formulaMmn+Rfp+AlSixOz where “M” is at least one metal cation having the weighted average valence “n”, “m” is the mole ratio of “M” to Al, “R” is at least one organoammonium cation having weighted average valence “p”, “r” is the ratio of “R” to Al, “X” is the mole ratio of Si to Al and is generally greater than or equal to 1, and “z” is the mole ratio of 0 to Al and is given byz=(m·n+r·p+3+4·x)/2.
Typical M cations include the alkali and alkaline earth cations Li, Na, K, Rb, Cs, Mg, Ca, Sr and Ba, as well as protons and ammonium cations, H+ and NH4+. Typical R organoammonium cations include quaternary ammonium cations such as diethyldimethylammonium, tetraethylammonium, tetramethylammonium, or protonated amines such as dimethylethylammonium, NMe2EtH+, or diethylammonium, NEt2H2+. Both the M and R cations are loosely bound to the structure and frequently can be completely or partially replaced with other cations by conventional ion exchange techniques. In the case of many R organoammonium cations, the species are too large to be removed by ion-exchange and calcinations or ammonia calcinations may be required to decompose the organic cation to form smaller protons or ammonium cations that more readily engage in ion-exchange processes. Currently over 200 species of both naturally occurring and synthetic zeolites are known.
Over the last 15 years at UOP LLC, Des Plaines, Ill., a series of novel zeolites have been developed that have been designated by the name UZM for UOP Zeolitic Material. Many of these materials have been prepared via the Charge Density Mismatch approach, which is disclosed in U.S. Pat. No. 7,578,993, which is incorporated by reference. This synthetic approach has enabled the isolation of zeolites with known framework topology but novel compositions, often higher Si/Al ratios or zeolites with completely new framework topologies. In the following discussion, the framework topologies of zeolites will be referred to by the 3-letter codes assigned to them by the Structure Commission of the International Zeolite Association (IZA). For reference, the frameworks and the codes can be viewed at the IZA website, http://www.iza-structure.org/databases/. UZM-4, disclosed in U.S. Pat. No. 6,419,895, has a high silica composition of the BPH framework topology. UZM-5, disclosed in U.S. Pat. No. 6,613,302, has a novel 2-dimensional framework, while UZM-7, disclosed in U.S. Pat. No. 8,158,104, has an unknown framework topology. UZM-8, disclosed in U.S. Pat. No. 6,756,030 is a layered zeolite, while UZM-9, disclosed in U.S. Pat. No. 6,713,041, is a high silica version of the LTA topology. UZM-10 is a high silica version of the OFF topology, while UZM-12, disclosed in U.S. Pat. No. 7,344,694, is a high silica version of the ERI topology. UZM-14, disclosed in U.S. Pat. No. 7,687,423, is a nanozeolite with the MOR topology; UZM-15, disclosed in U.S. Pat. No. 6,890,511, has an unknown framework topology, while UZM-16, disclosed in U.S. Pat. No. 6,752,980, is related to the OFF topology with some longer range features. UZM-22, disclosed in U.S. Pat. No. 7,744,850, has the MEI topology, while UZM-25, disclosed in U.S. Pat. No. 7,867,474 has the CDO topology. UZM-26, disclosed in U.S. Pat. No. 8,048,403, and UZM-27, disclosed in U.S. Pat. No. 7,575,678, are both new zeolites with currently unknown topologies. UZM-35, disclosed in U.S. Pat. No. 7,922,997, has a topology similar to MSE, and UZM-37, disclosed in U.S. Pat. No. 8,158,105, has a topology similar to MWW. Finally, UZM-45, disclosed in U.S. Pat. No. 8,597,611, has an unknown topology. The contents of these patents and patent applications referring to these UZM zeolites are hereby incorporated by reference. As disclosed in all of the individual patents, all these materials have ion-exchange capabilities. In many cases, this ion-exchange capability is enabled after the removal of organic cations by traditional ion-exchange, calcinations, or ammonia calcinations. Materials with ion-exchange capabilities may be used to remove undesirable metals from an aqueous stream, including soluble heavy metals such as Pb2+ and Hg2+. Applicants have found that the above mentioned UZM zeolites are effective for removing Hg2+ from aqueous streams.
Zeolites have previously been used to remediate Hg2+-containing aqueous streams. The use of natural zeolites is economically attractive for waste stream remediation. Ukrainian clinoptilolite has been used to remediate the Hg2+ waste stream from a Polish copper smelter, taking the effluent from 11 ppb to below 3 ppb as required by Polish law (See A. Chojnacki et al., MINERALS ENGINEERING (2004) 17, pp. 933-937.) However, less than 75% of the Hg2+ was removed in this instance. Another study of heavy metals removal using clinoptilolite showed that among the heavy metals, the selectivity for Hg2+ was the poorest (See JOURNAL OF HAZARDOUS MATERIALS, B97, (2003) pp. 219-243). Volcanic ash containing zeolites, known as green tuff, has been used as an agriculturally benign approach to remove heavy metals from aqueous streams (See JP 62059519A). An EPA study of Hg2+ removal looking at Y, Beta, FER, and MFI zeolites showed that a maximum of 75% of the Hg2+ could be removed from 10 ppm Hg2+ solutions, using large excesses of zeolite (10 ml solution/g zeolite), indicating a lack of selectivity (See S. Shevade, Mercury in the Environment: Managing and Assessing Multimedia Risks; Division of Environmental Chemistry, American Chemical Society, Preprints of extended abstracts, vol 42, No. 1, 851 (2002)). In US 20050181931, a zeolite-activated carbon composite is disclosed for water purification in which the zeolite may be pre-exchanged with some Ca2+ or Mg2+. The application claims the use of 4A, X and Y zeolites for this application to remove heavy metals, including Hg2+. Another study, citing the generally poor affinity of zeolites for Hg2+, treated natural clinoptilolite with the S-containing species cystamine and cysteamine in order to increase Hg2+ uptake, observing an improvement (See T. Gebremedhin-Haile et al., Water, Air and Soil Pollution, (2003) 148, pp. 179-200).
In contrast to the Hg2+ remediation efforts previously disclosed, applicants disclose a process for removing Hg2+ from an aqueous solution via ion-exchange using the zeolites of the UZM group enumerated above. High ion-exchange capacity is desirable for an ion-exchange material and for zeolites, this would point to highly charged, low Si/Al ratio species like 4A and X (Si/Al=1). Such zeolites are known to be very selective for Mg2+ and Ca2+, common constituents in water, which could interfere with or block the uptake of Hg2+. The UZM group of zeolites enumerated above fall into an intermediate range of Si/Al ratios between about 2-20 and usually 3-10. Applicants demonstrate that this UZM group of zeolites has appreciable capability for the uptake of Hg2+, much higher than high ion-exchange capacity zeolites.